The present invention relates to grinding apparatus with grinding discs which rotate relative to one another, defining therebetween a grinding space in which the material is ground under atmospheric or superatmospheric pressure and under corresponding temperature. The grinding discs are supported by an axially displaceable shaft or stator disc for adjustment of the spacing between the discs and which axial displacement is controlled by means of one or more servo motors. The grinding apparatus is principally intended for grinding lignocellulose-containing material in the form of chips or fiber products.
In order to achieve optimal grinding results, it is of great importance that the predetermined distance between the grinding discs is maintained constant during the grinding process, even in the event of variations in the amount of material to be ground. For example, the axial load on the grinding members can vary, for example, from zero tons at stoppage of the supply of material to 100 tons at full load of 25,000 kw.
During these axial load variations and with a fixed distance between the grinding elements in the range of 0.05 to 0.2 mm, depending upon the desired grinding result, it will be understood that extensions and retractions of the machine components which support the grinding elements may cause variations in the grinding space which exceed the pre-set value.
Variations in the grinding space defined between the two grinding elements under normal operation of a refiner are substantially linear with the axial load to which the grinding elements are subject.
This means that the space between the grinding elements can not be adjusted to the desired value during idling, but must be adjusted to the desired value during the actual grinding operation and at each change in the load factor.
In the event of sudden interruption in supply of material, the axial load is reduced to zero, with consequent neutralization of the extensions and retractions which are desired to be maintained in the apparatus, causing the spacing between the grinding elements to be immediately decreased to a degree where there will be frictional contact between the grinding discs.
Such frictional contact at a rotational speed on the order of 1,000 to 3,600 r.p.m. will cuase an immediate dry generated temperature increase up to the melting point of the grinding elements, with consequent destruction of the apparatus.
Several methods have been used heretofore in an attempt to prevent such destruction of the grinding elements. An example of these heretofore known methods is a load or feed sensor means which, for example, at decreased material supply or load, returns the grinding elements mechanically or hydraulically to a pre-determined position free of contact between the grinding discs. Several such systems are described in Swedish Patent No. 214,707, corresponding to U.S. Pat. No. 3,212,721 and in Swedish Patent No. 395,372 and corresponding U.S. Pat. No. 4,073,442 which describe single disc refiners having sensor means including an electrically controlled extension metering system or an electrically controlled resistance measuring system, or a mechanically controlled sensor or a hydraulically actuated wedge shaped member by means of which the spacing between the grinding elements is controlled.
The sensor means of the aforementioned prior art, although useful, may exhibit certain disadvantages. The electrical metering systems may not react in sufficient time to prevent contact between grinding elements in the event of a sudden interruption of supply material which unexpectedly reduces the axial load to zero, particularly where the refiner is already operating at relatively small pre-set grinding space between the grinding elements. As discussed in U.S. Pat. No. 4,073,442, the electrical sensor means first separate the grinding elements only after initial metallic contact between the two occurs. Additionally, the use of an elecrical metering increases the overall cost of the refiner apparatus as a result of the necessary electrical components and labor required to install the same, and increases the possibility of malfunction of the apparatus as a result of failure of the electrical sensor system.
The use of mechanical control means, such as the wedge-shaped element 92 described in U.S. Pat. No. 3,212,721, provides mechanical control means to displace a piston a distance corresponding to the relative displacement of the grinding elements which displacement corresponds to variations or deviations of the grinding space from its preset value. However, movement of the wedge element 92 is subject to frictional and inertial constraints. Accordingly, in operation, the displacement of the piston controlled by the wedge may not be of a continuous and dynamic nature, may not precisely correspond to the actual variation from the preset grinding space between grinding elements of the refiner, and may not react quickly enough to cause the necessary corrective action to be taken to return the grinding space to its preset value.